The present invention relates to an apparatus which is used to retain bone portions, such as vertebrae of a spinal column, in a desired spatial relationship.
A known apparatus for retaining vertebrae of a spinal column in a desired spatial relationship includes a longitudinal plate extendable along the spinal column. The plate includes a plurality of elongate slots extending along a longitudinal axis of the plate. A fastener has a first end portion that engages a vertebrae of the spinal column and a second end portion which extends through one of the slots in the plate. A nut threadably engages the second end portion of the fastener and clamps the plate to the fastener.
The elongate slots in the plate are defined by a plurality of frustoconical recesses which are engaged by a surface of the nut. The recesses define a plurality of nests which prevent movement of the plate relative to the fastener. The nests define a plurality of locations for receiving the fastener. Accordingly, there are only a certain number of positions for the fastener relative to the plate and infinite adjustments of the position of the plate relative to the fastener cannot be achieved.
The known longitudinal plates have a uniform stiffness along their length to prevent stress from being applied to healthy and deformed vertebrae to which the plate is connected. However, low stress deformation of bone is important in bone remodeling since the bone remodels to support applied loads. When the known plates are connected to bone, the surrounding bone of healthy vertebrae is shielded from applied stresses and the bone of the healthy vertebrae resorbs. Furthermore, the known stiff plates cause increased loads to be applied to a spine disk between the uppermost vertebra that the plate is connected to and an adjacent vertebra to which the plate is not connected. These increased loads cause degeneration of the disk.